1. Field of the Invention
The present invention relates to an image recording apparatus, such as a laser printer, LED printer, or liquid crystal printer, for recording an image on a recording medium by digitally applying a light beam.
2. Related Background Art
It is well known that there are apparatuses of the type in which a light beam is modulated in accordance with image information supplied from a computer or the like, and the modulated light beam is focused and scanned on a recording medium to record the image information, by using optical elements such as a light deflector and lens.
For such apparatuses, there are known two methods, one being an image exposure (IE) method which visualizes the area which was exposed to the light beam, and the other being a background exposure (BGE) method which visualizes the area which was not exposed to the light beam.
The IE method is more suitable for printing fine lines and characters, than is the BGE method.
An output image having a high image quality is desired, so a high quality image recording apparatus is now gradually becoming available, which has a recording density of 600 dpi to 1000 dpi.
However, the higher the recording density of such a light beam image recording apparatus becomes, the smaller the area of one pixel becomes, making it more difficult to reproduce such a small pixel.
FIGS. 2A and 2B show the light quantity distribution of a light beam having a diameter of about 100 .mu.m scanned at recording densities of 300 dpi and 600 dpi, respectively. The light beam diameter is defined as the diameter of the portion of a beam outside of which the light intensity lowers to 1/.sup.2 of the intensity at the beam center. In scanning the light beam, only one pixel (one dot) is made ON while the other pixels are made OFF, and on the other hand only one pixel is made OFF while the other pixels are made ON.
The abscissa represents a position in the subscanning direction, and the ordinate represents light quantity. The solid line indicates the light quantity when only one dot is recorded, and a broken line indicates the light quantity when only one dot is not recorded.
FIG. 2A shows the case of the recording density of 300 dpi, and FIG. 2B shows the case of the recording density of 600 dpi.
As seen from FIGS. 2A and 2B, in the case of the recording density of 300 dpi shown in FIG. 2A, the solid line intersects the broken line. Therefore, both one dot ON and one dot OFF are possible, because a certain light amount can be set as the recording threshold level. However, in the case of the recording density of 600 dpi shown in FIG. 2B, the solid line does not intersect the broken line. Therefore, although either one dot ON or one dot OFF is possible by setting a proper recording threshold level, to have both one dot ON and one dot OFF at the same time is impossible. This is true for both the IE and the BGE methods.
The reproducing ability at a high recording density can be improved if the light beam diameter is made small (e.g., in the case of 600 dpi, about 50 .mu.m). However, in order to make the light beam diameter so small, the optical system required is bulky and complicated, resulting in high cost. This problem becomes more serious as the recording density is made higher.
If such one pixel cannot be reproduced, the quality of an image including small characters or a halftone image using a dither method is degraded considerably.